The present invention relates to a device for determining the values of at least one parameter, especially the size, of particles such as water droplets.
Although not exclusively, said device is applicable more particularly to the detection and to the characterization of icing conditions on aircraft, especially civil transport airplanes, by making it possible to measure the size and the number of water droplets present in particular in the clouds and the fogs through which an aircraft passes.
It is known that the problem of rapid formation of ice on an aircraft is a serious problem which may lead to accidents when it occurs suddenly and when it could not be detected in time.
By means of documents U.S. Pat. No. 5,484,121, EP-0 405 625 and GB-2 158 939, devices are known for detecting the presence of ice on the external parts of an aircraft, especially the wings, once this ice is formed. These devices use, to this end, light beams and involve the reflection of these light beams in order to detect the presence of ice. If necessary, a signal is emitted in order to inform the pilot that it is necessary to activate a deicing device.
These known devices especially have the drawback of warning the pilot only when the ice is already formed. Therefore they do not allow the phenomenon of ice formation to be anticipated so that decisions suited to the climatic conditions can be taken before a problem appears.
Other devices are known for characterizing icing conditions, especially during development or test flights. It is known that, during development flights, it must be demonstrated that the conditions encountered comply properly with the regulatory requirements decreed by the certification authorities, which then validates the behavior of the airplane under such conditions.
The known devices, used to this end, are generally intended to determine the particle size distribution of clouds, that is to say the size, the number and the shape of the water droplets present in these clouds, and they can be classified in two categories:
particle counters which determine the characteristics of each particle, taken individually; and
integrators which sum the contributions from all the particles present in a measuring volume.
These known devices may also be classified according to their measurement principle, namely in particular:
imaging or shadowgraphy,
the Doppler effect, or
light scattering (measurement along given angles).
By way of illustration, it is possible to mention some examples of known devices implementing imaging and shadowgraphy principles and acting, for example, as particle counters.
Firstly, a device which is known by the term xe2x80x9cOptical Array Probexe2x80x9d implements, in a standard manner, the principle called linear array shadowgraphy. In this case, a particle passing through a collimated light beam produces a shadow directly related to its diameter on a linear array of detectors spaced apart at equal intervals. When there is no particle in the measuring volume in question, the light beam illuminates all the detectors. On the other hand, when a particle passes through said measuring volume, a loss of light due to scattering, refraction and absorption of the particle generates a signal at the output of the detectors. The number of detectors showing a variation in amplitude of more than a specific threshold (for example 50%) is summed in order that the size of the particle is given directly. However, this known device has a limited measurement range and is not able to measure particles accurately, if their diameter is less than 100 xcexcm.
Secondly, a device which is known by the term xe2x80x9cCloud Particle Imagerxe2x80x9d generates two laser beams. The intersection of these two lasers beams defines a rectangular sampling area. Any particle which passes through this rectangular sampling area is properly focused and actuates illumination by an imaging laser, for the purpose of acquiring an image. Detectors are placed facing the laser beams: they make it possible to detect the passage of particles by measuring the decrease in intensity produced as these particles pass through. The diameter of the particles is measured from the image of the properly focused particle. However, this known device is bulky and has too large a volume to improve the operational difficulties stated above and linked to the devices currently used in in-flight testing.
Thirdly, a measuring device is known by document FR-2 689 247, in particular comprising:
a rod comprising a measuring region which is intended to accommodate the particles to be analyzed;
illumination means capable of illuminating said measuring region, using at least one laser beam;
image acquisition means capable of acquiring images of said measuring region illuminated by said illumination means; and
processing means capable of determining the values of said parameter, from said images.
More specifically, the latter known device emits a pulsed light beam that is transported by optical fiber and which is focused onto the measuring region. The image is also transported by optical fiber up to a beam splitter which divides and orients the beam toward the image acquisition means comprising two xe2x80x9cCCDxe2x80x9d-type sensors. A first image is recorded along a particular sighting axis on one of the sensors. After a very short time, a second image is recorded along the same sighting axis on the other sensor. On subtracting these two images, a dark/light doublet is obtained which stands out well against the uniform background, from which it is possible to deduce the size and the velocity of the imaged particles. This known device makes it possible to remove the majority of background defects. This is because the image acquisition means with a double sensor behave like a double shutter and only see the field for two very short periods of time.
However, this known device has a small measuring volume. It is known that the measuring volume analyzed per second is equal to the measuring volume associated with each image, multiplied by the image rate of the image acquisition means.
As to the measuring volume associated with each image, this is related to the size of the sensor (divided by the magnification) and to the depth of field.
Consequently, in particular because of its small measuring volume, the known device disclosed by document FR-2 689 247 is not suitable for the applications envisaged in the present invention, relating to the measurement of water droplet parameters since, especially because of the often relatively low concentrations of water droplets present in the clouds analyzed during tests, a large measuring volume is needed for such an application.
It will be noted, moreover, that the various known devices comprise various drawbacks which are problematic for the aforementioned preferred application. This is because the known devices are, in general, poorly adapted to the envisaged meteorological and operational constraints, in particular because of the following difficulties, namely a long and difficult installation in an aircraft, a very large bulk, difficult exploitation of the results, etc. Furthermore, the majority of these known devices have a range for measuring of the size of the droplets which is small, and especially are not able to detect and analyze, at the same time, the small droplets and the large supercooled droplets (water at a temperature less than 0xc2x0 C.) which, as is known, promote the appearance of ice.
The present invention relates to a device for determining the values of at least one parameter, especially the size, of particles, in particular of water droplets, which makes it possible to overcome the aforementioned drawbacks and which especially comprises an increased measuring volume.
To this end, according to the invention, said device of the type comprising:
a measuring element comprising a measuring region which is intended to accommodate the particles;
illumination means capable of illuminating said measuring region;
image acquisition means comprising at least one camera capable of acquiring at least one image of said measuring region illuminated by said illumination means; and
processing means capable of determining the values of said parameter, from said image acquired by the camera, is noteworthy in that said illumination means are constituted so as to produce a point illumination source using a light beam, the light rays of which are focused on an objective optic of the image acquisition means.
Thus, by means of this point illumination, the contrast (on the image or images acquired) of the shadow of the particles located in the measuring region is increased. This increase in contrast leads to an improved observability of the particles at the expense of focus and therefore an increase in the depth of field, since the image remains observable for higher defocused values. This increase in depth of field itself leads to an increase in the measuring volume which, as indicated above, depends on the size of the sensor and on the depth of field.
In a preferred embodiment, in order to produce said point illumination, said illumination means comprise at least one optical assembly comprising:
a light source, preferably a laser source, capable of generating a light beam;
an optical fiber connected by a first end to said light source and capable of transmitting a light beam generated by the latter; and
a field optic fitted to a second end opposite said first end of said optical fiber and focusing the light beam emerging from said optical fiber onto the center of the objective optic of the camera of said image acquisition means.
Preferably, said optical fiber is a monomode fiber, that is to say a fiber which, by construction, makes it possible to transmit only a single mode of a laser beam. This makes it possible to prevent the appearance of unwanted noise.
Furthermore, in order to yet further increase the measuring volume, in an advantageous manner:
said illumination means comprise a light source generating a coherent light beam, which makes it possible to increase the aforementioned contrast; and/or
a camera is used which has a high image acquisition rate; and/or
said camera comprises means which open the latter in order to acquire an image, said illumination means produce illumination in the form of light flashes, and are controlled so as to emit at least two light flashes during one and the same opening of the camera on acquiring an image. Thus, the measuring volume which is observed during the camera opening period is increased.
In order to implement the last characteristic, preferably, said illumination means comprise a pulsed laser with a saturable absorber in order to emit said light flashes, that is to say a laser making it possible to emit light flashes of very short durations. This makes it possible to compensate for the flow velocity of the particles. In effect, the light flash freezes the particles.
Moreover, in order to increase the size range of measured particles, especially of water droplets, in an advantageous manner, said illumination means are constituted so as to emit in a sequential manner at least two light beams dedicated to measuring different diameters.
In order to do this, in a preferred embodiment, said illumination means comprise two laser sources associated with optics of different magnifications respectively, and said image acquisition means comprise a single camera and optical means making it possible to direct the two laser beams emitted by said two laser sources, onto said camera.
In order to yet further increase the measurement range, said processing means are constituted so as to determine said parameter by shadowgraphy.
Moreover, advantageously, said illumination means are constituted so as to emit two light flashes spaced apart by a predetermined duration, said camera is constituted so as to acquire an image on emission of each of said light flashes, and said processing means are constituted so as to determine, as a parameter, the velocity vector of said particles, from the superposition of two images relating to said two light flashes and from said predetermined duration.
To this end, said illumination means preferably comprise a light source and an optical fiber which is connected to said light source.
In addition, advantageously, the device according to the invention further comprises:
an interference filter which is fitted to the entrance of said camera, in order to filter out the unwanted light which might reach the camera; and/or
an antireflection treatment in order to prevent interference; and/or
a measuring region which is delimited by at least one window; and/or
means to blow air, preferably filtered air, over the external face of said window, which prevents the appearance of dirt on said window.
In a preferred embodiment, said measuring element is a rod provided with a through opening, preferably oblong, at a first of its ends, said through opening containing said measuring region.
In addition, advantageously, the device according to the invention also comprises a protection specified below, which protects the entire said device, except for at least said first end of the rod which is itself placed directly into the environment containing said particles, against said particles which are generally moving.
In addition, advantageously, at least said first end of the rod is electrically insulated so as to make the device according to the invention unattractive to lightning, for example, if said device is fitted onto an aircraft, as specified below.
Moreover, it will be noted that the device according to the invention is particularly well suited to provide certification authorities with reliable quality information on the conditions encountered by an aircraft during in-flight tests. Of course, there may be other possibilities of use thereof, such as the certification of helicopters during stationary flights, the verification of conditions produced in icing wind tunnels, measurements on the ground during poor conditions of visibility (fog, etc.), etc.
Another beneficial application relates to the prevention and anticipation of icing conditions on an aircraft. Specifically, as indicated above, the majority of existing devices only warn the pilot once the ice is established on the sensitive regions of the aircraft. In contrast, the understanding of the conditions which lead to the formation of ice by studying droplets which form the cloud, combined with temperature information, makes it possible to anticipate this ice formation. Thus, by means of a device according to the invention which makes it possible to study water droplets over the whole measurement range involved, it is possible to warn the pilot of the probable formation of ice before the latter is formed on the sensitive regions of the aircraft.
The present invention also relates to an aircraft, in particular a civil transport airplane, which is noteworthy in that it comprises a device as mentioned above, for determining the values of said parameter of water droplets present outside said aircraft.
Advantageously, said aircraft further comprises a sleeve:
which is fitted into the fuselage of the aircraft so as to create a through opening;
which has a diameter adapted to the diameter of the aforementioned rod of the device according to the invention such that the latter can be fitted in a sealed manner in said sleeve; and
which is capable of being closed in a sealed manner, in the absence of said rod.
Thus the device according to the invention can be mounted and dismantled easily and quickly. In addition, the aerodynamics of the aircraft are only slightly altered.